1. Field of the Invention
The present invention relates to a spark-ignition engine which burns an air-fuel mixture in a combustion chamber by compression ignition in a part-load range under warm-running conditions.
2. Description of the Related Art
Compression ignition has intensively been studied in recent years as means for improving fuel economy in spark-ignition engines (gasoline engines). The compression ignition is spontaneous firing of a mixture in a combustion chamber occurring under high temperature and pressure conditions in a final part of a compression stroke as in a diesel engine. As the compression ignition causes the mixture to rapidly burn throughout the entire combustion chamber, the fuel economy greatly improves as a result of increased combustion efficiency. In addition, the compression ignition serves to decrease the production of nitrogen oxides (NOx), contributing thereby to an improvement in emissions.
Prior art proposes an arrangement for leaving hot burned gas in the combustion chamber by so-called internal exhaust gas recirculation (internal EGR) as a technique for producing the compression ignition. In an engine disclosed in Japanese Unexamined Patent Publication No. 2001-152919, for example, an intake valve is caused to open after an exhaust valve has closed to establish a negative overlap period, during which both of the intake valve and the exhaust valve are closed, in a specific part-load operation region, so that the amount of burned gas left in the combustion chamber due to the internal EGR increases and the compression ignition occurs as a result of a temperature increase in the combustion chamber caused by the internal EGR.
Although the compression ignition is efficiently produced due to the temperature increase in the combustion chamber by the internal EGR in a relatively low-speed, low-load operating region according to the aforementioned conventional arrangement, knocking is likely to occur under high-speed, high-load conditions due to an excessive temperature increase in the combustion chamber. In regions of increased knocking tendency, the internal EGR and compression ignition are interrupted and the mixture is ignited by an electric spark with the intake valve and the exhaust valve opened and closed with normal timing. It is however desirable to broaden a range of combustion initiated by compression ignition for improving the fuel economy and the quality of emissions. There has been a room for improvement in conventional spark-ignition engines from this point of view.